Current evidence suggests that sexual size dimorphism (SSD) reflects the male and female adaptation to their different reproductive roles. Belostoma and Lethocerus species, included in Belostomatidae, present different kinds of paternal care. Females of Belostoma Latreille species lay their eggs on the back of males. Males carry, aerate and protect the clutch until hatching, which is critical for offspring survival. Males of Lethocerus Mayr species exhibit some parental care behavior but do not carry the eggs. The genera are nearly related. We studied and compared the SSD patterns of B. oxyurum (Dufour), B. micantulum (Stål), B. elegans (Mayr), B. bifoveolatum Spinola, B. gestroi Montandon and Lethocerus annulipes (Herrich-Schäffer) by means of a multivariate approach to distinguish selection targets in different components of size. Morphometric analysis revealed that SSD patterns vary among traits and that the arrangements are similar in Belostoma species, showing a common trend under resembling selective mechanisms. The widespread SSD trend in insects is that all components of body size are biased towards females, generally related to a fecundity advantage, a pattern now also detected in L. annulipes. We found in Belostoma species that the male has relatively longer middle and hind legs. We propose that SSD in hind legs biased towards males is a selective response for paternal care; they denote a brood-adapted morphology. The middle leg enlargement may be an associated response to maintain effective locomotion.